1. Field of the Invention
The present invention relates to a method of manufacturing a liquid crystal display and more particularly to a method of manufacturing a color liquid crystal display having coloring layers on pixel electrode portions.
2. Background Description
As a method of manufacturing a liquid crystal display, the following method is disclosed in Tsujimura et al., xe2x80x9cSelf Align Patterning Methodxe2x80x9d in IBM Technical Disclosure Bulletin, RD v41 n409 05-98 article 40991. In this method, a transparent pixel electrode, e.g., an ITO (Indium-Tin-Oxide) electrode is formed on a TFT (Thin Film Transistor) array structure by back side exposure using a negative resist. FIGS. 1a to 1d are diagrammatic cross sectional views of a process of forming the ITO electrode by this known back side exposure method. First, a gate electrode 2 is patterned on a transparent substrate 1 made of glass. A gate insulating film 3, an amorphous silicon (a-Si) film 4 and an etching protective film 5 are deposited. The etching protective film 5 is patterned. An n+a-Si film and source and drain electrode films are deposited. Source and drain electrodes 6 and 7 and a data line 8 are patterned. The n+a-Si film is etched. The structure, in which a thin film transistor (TFT) structure and the data line are formed on the transparent substrate 1 as shown in FIG. 1a, is thus obtained. Then, as shown in FIG. 1b, the structure of FIG. 1a is coated with an interlayer polymer resin and the resin is exposed to a light, developed and baked, whereby an interlayer polymer resin layer 9 is formed. An ITO conductive film 10 is formed over the interlayer polymer resin layer 9. The ITO conductive film 10 is coated with a negative resist 11. Then, the negative resist 11 is exposed to the light from a light source, e.g., an ultra-high pressure mercury lamp from a back side of the transparent substrate 1. In this case, the gate line and the data line 8, which are an opaque metal, are used as a photomask. Then, the negative resist 11 is developed and baked, so that an exposed portion 11xe2x80x2 alone of the negative resist 11 remains as shown in FIG. 1c. This portion 11xe2x80x2 partially overlaps the gate line and the data line used as the photomask, due to diffraction of light which occurs at the time of the back side exposure. Then, the ITO conductive film 10 is etched by a mixed liquid of nitric acid and hydrochloric acid and thus the negative resist 11xe2x80x2 is removed, whereby an ITO electrode 10xe2x80x2 is obtained as shown in FIG. 1d. 
According to such a back side exposure method, it is possible to equalize an overlap length of the ITO electrode and the data line resulting from the diffraction of light which occurs at the time of the back side exposure. Thus, capacities of the ITO electrode and the data line can be equalized. Thus, deterioration in display quality due to vertical crosstalk does not occur. Moreover, there is no problem of a surface seam resulting from stepper exposure. Thus, a thickness of polymer can be reduced, and therefore the process is facilitated. Furthermore, since the ITO electrode overlaps the data line, a horizontal electric field is not applied to liquid crystal molecules on the ITO electrode. Thus, a discrimination line remains on the data line alone. Therefore, it is not necessary to hide the discrimination line. Consequently, an opening ratio is increased.
When such a back side exposure method is used in a so-called CFA (Color Filter on Array) structure, the method in which pigment-dispersed red, green and blue color resist layers are buried under the polymer resin layer as a coloring layer as shown in FIG. 2a has an advantage in reliability of a panel.
When the back side exposure is performed by using the ultra-high pressure mercury lamp, exposure energy of the negative resist on each color pixel is expressed as the following equation:
Dosered=tM(xcex)G(xcex)Cred(xcex)N(xcex)dxcex;
Dosegreen=tM(xcex)G(xcex)Cgreen(xcex)N(xcex)dxcex;
and
Doseblue=tM(xcex)G(xcex)Cblue(xcex)N(xcex)dxcex,
where t represents an exposure time, M(xcex) represents an emission spectrum of the ultra-high pressure mercury lamp, G(xcex) represents a transmission spectrum of the glass, C(xcex) represents the transmission spectrum of each color resist, and N(xcex) represents an absorption spectrum of the negative resist. It is seen that the exposure energy of the negative resist on each pixel depends on the transmission spectrum of each color resist.
FIG. 3 is a graph of a relationship between the transmission spectrum of each color resist and the negative resist and the spectrum of the ultra-high pressure mercury lamp. The ultra-high pressure mercury lamp for typical use as the light source has sharp peaks of an i line (365 nm), a g line (405 nm) and an h line (436 nm). Thus, a shape of each spectrum near each of these lines becomes a problem. As can be seen from this graph, the red, green and blue color resists have substantially the same transmission intensity near the i line. On the other hand, the color resists considerably differ in transmission intensity near the g line and the h line. That is, the transmission intensity of the blue color resist is very high, and the transmission intensity of the red color resist is higher than that of the green color resist. FIG. 4 is a graph of the transmission intensity of each of other various color resists. It is seen from this graph that the transmission intensity differs depending on the color of the color resist.
Transmission properties differ depending on the color of the color resist buried in the pixel. Thus, the overlap of the ITO electrode and the data line resulting from the diffraction of light varies depending on the color of each color resist as shown in FIGS. 2b and 2c. FIG. 5 shows an electron photomicrograph of the pixel electrode formed by the above-described conventional back side exposure method. In this example, the following fact is seen. The negative resist of a green pixel is sufficiently exposed to the light. Thus, the overlap of the ITO electrode and the data line of a blue pixel is greatly increased. A short circuit occurs between adjacent pixels. Thus, a difference in the overlap length of the ITO electrode and the data line depending on the color of the color resist manifests itself in the form of blue crosstalk, for example, and deteriorates the display quality.
The following fact is disclosed in Japanese Patent Publication No. 7-104516. A positive resist is exposed to the light of 400 nm or less, whereby a high-accuracy transparent electrode pattern can be formed on a color filter by self alignment using the color filter as the photomask. The self alignment can be executed by using as the photomask the color filter whose transmittance is about 0 at 400 nm or less as shown in FIG. 3.
It is an object of the present invention to provide a method of manufacturing a color liquid crystal display capable of equalizing an overlap length of an ITO electrode and a data line regardless of colors of color resists buried in a pixel.
It is another object of the present invention to provide a method of manufacturing a color liquid crystal display capable of equalizing an overlap length of the ITO electrode and a data line regardless of colors of color resists buried in a pixel, while forming the ITO electrode by back side exposure using a negative resist.
The present invention is a method of manufacturing a color liquid crystal display according to the present invention comprises the steps of: forming coloring layers composed of a plurality of colors on a transparent substrate on which a thin film transistor structure, a gate line and a data line are formed; forming a transparent conductive film over the entire transparent substrate on which the coloring layers are formed; coating the entire surface of the transparent conductive film with a negative resist; exposing the negative resist to a light using the gate line and the data line as a photomask, the light being emitted from a light source facing a back side of the transparent substrate, the light substantially having wavelength bands excluding 390 nm to 440 nm; developing and baking the exposed negative resist; and etching and removing the transparent conductive film in a portion where the negative resist is removed.